Electrode for secondary battery and secondary battery

ABSTRACT

An electrode using a carbon nanotube as a conductive material, and excellent in resistance characteristics is provided. An electrode for a secondary battery herein disclosed has a collector, and an active material layer formed on the collector. The active material layer includes an active material and a carbon nanotube. At least a part of the surface of the carbon nanotube is coated with a material including an element with a lower electronegativity than that of carbon.

It should be noted that the present application is a divisional of U.S.patent application Ser. No. 16/525,817 filed on Jul. 30, 2019, whichclaims the benefit of priority based on Japanese Patent Application No.2018-152283 filed on Aug. 13, 2018, the content of which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present teaching relates to an electrode for a secondary battery.The present teaching also relates to a secondary battery including theelectrode.

2. Description of the Related Art

In recent years, a secondary battery such as a lithium ion secondarybattery has been suitably used for a portable power source for apersonal computer, a portable terminal, or the like; a vehicle drivingpower source for an electric vehicle (EV), a hybrid vehicle (HV), aplug-in hybrid vehicle (PHV), or other vehicles; or the like.

An electrode for use in a secondary battery such as a lithium ionsecondary battery typically has a configuration in which an activematerial layer is provided on a collector. A technology of incorporatinga conductive material into an active material layer in order to enhancethe electron conductivity of the active material layer has been known.

Japanese Patent Publication No. 6136788 describes that use of a carbonnanotube in a small amount as a conductive material can improve theelectron conductivity of the active material layer.

SUMMARY OF THE INVENTION

Further improvement in resistance characteristics such as a decrease inresistance has been required for secondary batteries such as lithium ionsecondary batteries. For such a requirement, the present inventor hasconducted intensive studies thereon, and as a result, the presentinventor has found that, in the conventional art, a certain level ofresistance reducing effect can be achieved by the electron conductivityimproving effect due to carbon nanotubes, but there is still room forimprovement of the resistance characteristics.

Under such circumstances, it is an object of the present teaching toprovide an electrode using a carbon nanotube as a conductive material,and excellent in resistance characteristics.

The electrode for a secondary battery herein disclosed includes acollector, and an active material layer formed on the collector. Theactive material layer includes an active material and a carbon nanotube.At least a part of a surface of the carbon nanotube is coated with amaterial including an element with a lower electronegativity than thatof carbon.

Such a configuration provides an electrode using a carbon nanotube as aconductive material, having a small initial resistance, and suppressedin an increase in resistance after high-temperature storage. Namely, anelectrode using a carbon nanotube as a conductive material, andexcellent in resistance characteristics is provided.

In accordance with one desirable aspect of the electrode for a secondarybattery herein disclosed, the element with a lower electronegativitythan that of carbon may be at least one selected from the groupconsisting of Ti, P, B, Si, Al, Zn, and W.

With such a configuration, the resistance characteristics improvingeffect becomes particularly high.

In accordance with one desirable aspect of the electrode for a secondarybattery herein disclosed, the electrode for a secondary battery may be apositive electrode, and the active material may be a positive electrodeactive material.

With such a configuration, the initial resistance reducing effectbecomes particularly high.

In accordance with one desirable aspect of the electrode for a secondarybattery herein disclosed, the electrode for a secondary battery may be anegative electrode, and the active material may be Li₄Ti₅O₁₂ or Si.

With such a configuration, the initial resistance reducing effectbecomes particularly high.

In accordance with one desirable aspect of the electrode for a secondarybattery herein disclosed, the average length of the carbon nanotubes maybe 3 μm or more and 50 μm or less.

With such a configuration, the resistance characteristics improvingeffect becomes particularly high.

In accordance with one desirable aspect of the electrode for a secondarybattery herein disclosed, the coating ratio of the surface of the carbonnanotube with the material including an element with a lowerelectronegativity than that of carbon may be 40% or more and 70% orlower.

With such a configuration, the resistance characteristics improvingeffect becomes particularly high.

In accordance with one desirable aspect of the electrode for a secondarybattery herein disclosed, the material including an element with a lowerelectronegativity than that of carbon may be an oxide of the elementwith a lower electronegativity than that of carbon.

With such a configuration, the element with a lower electronegativitythan that of carbon becomes less likely to be eluted from the coating,resulting in an improvement of the durability of the coating.

The secondary battery herein disclosed includes the electrode for asecondary battery described above.

Such a configuration provides a secondary battery excellent inresistance characteristics (i.e., having a small initial resistance, andsuppressed in an increase in resistance after high-temperature storage).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view showing a part of an exampleof an electrode in accordance with one embodiment of the presentteaching;

FIG. 2 is a cross sectional view schematically showing a configurationof a lithium ion secondary battery constructed using an electrode inaccordance with one embodiment of the present teaching; and

FIG. 3 is a schematic view showing a configuration of a wound electrodebody of a lithium ion secondary battery constructed using an electrodein accordance with one embodiment of the present teaching.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present teaching will be described below. It shouldbe noted that matters necessary for executing the present teaching,except for matters specifically referred to herein (e.g., a generalconfiguration of an electrode not featuring the present teaching) can berecognized as design matters of those skilled in the art based on therelated art in the present field. The present teaching can be executedbased on the contents disclosed herein, and the technical common sensein the present field. Further, in the following accompanying drawings,the members and portions exerting the same function are given the samereference number and sign for description. Further, the dimensionalrelation (such as length, width, or thickness) in each drawing does notreflect the actual dimensional relation.

In the present description, “secondary battery” is a term denoting anelectric storage device capable of repeatedly charging and dischargingin general, and including so-called electric storage elements such as astorage battery and an electric double layer capacitor.

Hereinafter, an embodiment in which the electrode for a secondarybattery herein disclosed is an electrode for a lithium ion secondarybattery will be specifically described with reference to theaccompanying drawings.

FIG. 1 is a schematic cross sectional view showing a part of theelectrode in accordance with the present embodiment. As shown in FIG. 1,an electrode 10 in accordance with the present embodiment has acollector 12, and an active material layer 14 formed on the collector12.

The collector 12 is typically a member made of a metal with goodconductivity. As the collector 12, for example, a sheet-shaped membersuch as metal foil, a metal mesh, or a punching metal can be used.

When the collector 12 is a positive electrode collector, the collector12 is desirably a member made of aluminum or aluminum alloy, and moredesirably aluminum foil.

When the collector 12 is a negative electrode collector, the collector12 is desirably a member made of copper or copper alloy, and moredesirably copper foil.

The active material layer 14 includes an active material 16.

When the active material 16 is a positive electrode active material,examples thereof may include lithium transition metal oxides (e.g.,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNiO₂, LiCoO₂, LiFeO₂, LiMn₂O₄, orLiNi=_(0.5)Mn_(1.5)O₄), and lithium transition metal phosphate compounds(e.g., LiFePO₄).

When the active material 16 is a negative electrode active material,examples thereof may include carbon materials such as graphite, hardcarbon and soft carbon; lithium titanate (Li₄Ti₅O₁₂: LTO); and Si, andSn.

Since the initial resistance reducing effect is particularly high, theelectrode 10 in accordance with the present embodiment is desirably apositive electrode, and the active material 16 is desirably a positiveelectrode active material (particularly, a lithium transition metaloxide).

Because of the particularly high initial resistance reducing effect,desirably, the electrode 10 in accordance with the present embodiment isa negative electrode, and the active material 16 is LTO or Si.

It should be noted that the electrode 10 in accordance with the presentembodiment herein described is for a lithium ion secondary battery, butwhen the electrode 10 is formed as an electrode for other secondarybatteries, the kind of the active material may be appropriatelyselected.

The active material layer 14 includes a carbon nanotube 18.

As the carbon nanotube 18, for example, a single-walled carbon nanotube(SWNT) or a multilayer carbon nanotube (e.g., double-walled carbonnanotube (DWNT)), or the like can be used. These can be used singly, orin combination of two or more thereof.

The carbon nanotube 18 may be a product manufactured by an arc dischargemethod, a laser ablation method, a chemical vapor deposition method, orthe like.

The average length of the carbon nanotubes 18 is not particularlyrestricted. When the average length of the carbon nanotubes 18 is short,the conductive path between the active materials tends to become lesslikely to be formed. For this reason, the average length of the carbonnanotubes 18 is desirably 1 μm or more, more desirably 2 μm or more, andfurther desirably 3 μm or more. Meanwhile, when the average length ofthe carbon nanotube 18 is large, the carbon nanotubes 18 are aggregated,and are not uniformly dispersed. Accordingly, the electron conductivityimproving effect tends to be less likely to be obtained. For thisreason, the average length of the carbon nanotubes 18 is desirably 100μm or less, more desirably 75 μm or less, and further desirably 50 μm orless. Since the resistance characteristics improving effect isparticularly high, the average length of the carbon nanotubes 18 is mostdesirably 3 μm or more and 50 μm or less.

The average diameter of the carbon nanotubes 18 is not particularlyrestricted, and is desirably 0.1 nm or more and 30 nm or less, and moredesirably 0.5 nm or more and 20 nm or less.

The average length and the average diameter of the carbon nanotubes 18can be determined, for example, as the average values of the lengths andthe diameters of 30 or more carbon nanotubes 18, respectively, by takingthe electron micrograph of each carbon nanotube 18.

In the present embodiment, at least a part of the surface of the carbonnanotube 18 is coated with a material including an element with a lowerelectronegativity than that of carbon. In other words, the carbonnanotube 18 has a coating (not shown) of a material including an elementwith a lower electronegativity than that of carbon on at least a part ofthe surface thereof.

The elements with a lower electronegativity than that of carbon aredesirably B, P, and a metal element.

Specific examples of the element with a lower electronegativity thanthat of carbon may include Ti (1.54), P (2.19), B (2.04), Si (1.90), Al(1.61), Zn (1.65), and W (2.36). Out of these, at least one selectedfrom the group consisting of Ti, P, B, Si, Al, Zn, and W is desirablebecause of their particularly high resistance characteristics improvingeffects. It should be noted that the numerals indicated together withthe symbol of element means values of the electronegativity of theelement.

A smaller electronegativity of the element tends to provide a largerresistance reducing effect. For this reason, the electronegativity ofthe element is desirably 2.4 or lower, more desirably 2.0 or lower, andfurther desirably 1.8 or lower.

The material including an element with a lower electronegativity thanthat of carbon may include only one element with a lowerelectronegativity than that of carbon, or may include two or morethereof. When the material includes two or more elements,non-homogeneity occurs in the distribution of the elements in thecoating film, which may improve the characteristics.

The material including an element with a lower electronegativity thanthat of carbon may be composed of one element, or may be composed of twoor more elements.

The material including an element with a lower electronegativity thanthat of carbon may include an element with a higher electronegativitythan that of carbon within the range not to impair the effects of thepresent teaching. Examples of the element with a higherelectronegativity than that of carbon may include O (3.44).

Accordingly, for example, the material including an element with a lowerelectronegativity than that of carbon may be an oxide of an element witha lower electronegativity than that of carbon. In this case, the elementwith a lower electronegativity than that of carbon becomes less likelyto be eluted from a coating, resulting in an improvement of thedurability of the coating.

The coating may include a binder such as acrylic resin.

The active material layer 14 may include other components than theactive material 16 and the carbon nanotube 18. Examples thereof mayinclude a binder and a thickener.

As the binder, for example, various polymer materials such aspolyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE),polyethylene oxide (PEO), and styrene butadiene rubber (SBR) can beused.

As the thickener, for example, carboxymethyl cellulose (CMC) can beused.

The active material layer 14 may include conductive materials (e.g.,carbon black such as acetylene black (AB)) except for the carbonnanotube 18 within the range not impairing the effect of the presentteaching.

The coating ratio (i.e., coverage rate) of the surface of the carbonnanotube 18 with a material including an element with a lowerelectronegativity than that of carbon is not particularly restricted.When the coverage rate is too low, the resistance characteristicsimproving effect due to coating tends to be reduced. For this reason,the coverage rate is desirably 20% or more, more desirably 30% or more,and further desirably 40% or more. On the other hand, when the coveragerate is too high, the carbon nanotube 18 functioning as the conductivepath may be insulated. For this reason, the coverage rate is desirably90% or lower, more desirably 80% or lower, and further desirably 70% orlower. The coverage rate is most desirably 40% or more and 70% or lowerbecause of the particularly high resistance reducing effect.

It should be noted that the coverage rate of the carbon nanotube 18 canbe determined, for example, in the following manner. First, using anenergy dispersion type X ray analysis device (EDS device), across-section EDS mapping image is acquired, and the element depositedat the carbon nanotube 18 is identified. Then, in the cross-section EDSmapping image, (total of the deposition distances of element(s))/(entirecircumferential length of carbon nanotube 18)×100 is calculated. Thecalculated value can be referred to as a coverage rate (%).Alternatively, the coverage rate (%) can also be determined using anelectron probe micro analyzer (EPMA) in the same manner.

The carbon nanotube 18 coated with a material including an element witha lower electronegativity than that of carbon on at least a part of thesurface thereof can be manufactured, for example, in the followingmanner.

For example, mention may be made of a method in which vapor depositionis performed while rotating the carbon nanotube. With this method, thesurface of the carbon nanotube can be coated with only an element with alow electronegativity.

Alternatively, for example, mention may be made of the following method:a dispersion of a material including an element with a lowerelectronegativity than that of carbon (particularly, an oxide of anelement with a lower electronegativity than that of carbon) having aparticle diameter of nano order is first prepared, a carbon nanotube isthen immersed in the dispersion, and the dispersion medium of thedispersion is dried and removed. For the purpose of improving thestrength of the coating, a binder such as an acrylic resin may beallowed to be contained in the dispersion.

In the present embodiment, the carbon nanotube 18 coated with a materialincluding an element with a lower electronegativity than that of carbonon at least a part of the surface thereof is used as a conductivematerial of the active material layer 14. This can provide an electrode10 excellent in resistance characteristics. Specifically, the electrode10 in accordance with present embodiment has an effect of a smallinitial battery resistance. In addition, the electrode 10 has an effectof being largely suppressed in increase in battery resistance even afterhigh-temperature storage.

The reason for this is presumed as follows.

When the carbon nanotube 18 has a coating of a material including anelement with a lower electronegativity than that of carbon, the surfaceof the coating is positively charged, and attracts electrons. As aresult, the electron conductivity of the carbon nanotube 18 is improved,and the battery resistance is reduced. Further, the surface of thecoating becomes basic, and traps the acid (e.g., HF) generated bydecomposition of a non-aqueous electrolytic solution, or the like. Thissuppresses the increase in battery resistance during high-temperaturestorage by the acid.

The content of the active material 16 is not particularly restricted,but is desirably 75 mass % or more, more desirably 80 mass % or more,and further desirably 85 mass % or more in the active material layer 14(i.e., based on the total mass of the active material layer 14).Meanwhile, the content of the active material is desirably 99.8 mass %or lower, more desirably 99.5 mass % or lower, and further desirably 99mass % or lower in the active material layer 14.

The content of the carbon nanotube 18 is not particularly restricted,and is desirably 0.1 mass % or more, more desirably 0.3 mass % or more,and further desirably 0.5 mass % or more in the active material layer14. Meanwhile, the content of the carbon nanotube 18 is desirably 15mass % or lower, more desirably 10 mass % or lower, and furtherdesirably 5 mass % or lower in the active material layer 14.

The content of the binder is not particularly restricted, and isdesirably 0.1 mass % or more and 10 mass % or lower, and more desirably0.5 mass % or more and 8 mass % or lower in the active material layer14.

The content of the thickener is not particularly restricted, and isdesirably 0.1 mass % or more and 5 mass % or lower, and more desirably0.5 mass % or more and 3 mass % or lower in the active material layer14.

The electrode 10 in accordance with the present embodiment is for asecondary battery, desirably for a non-aqueous electrolytic solutionsecondary battery, and particularly desirably for a lithium ionsecondary battery. In accordance with a known method, using theelectrode 10 in accordance with the present embodiment, a secondarybattery (particularly, a non-aqueous electrolytic solution secondarybattery) can be constructed. The secondary battery is excellent inresistance characteristics. Specifically, the secondary battery has asmall initial resistance, and is excellent in input outputcharacteristics. Further, the secondary battery is suppressed inincrease in resistance after high-temperature storage, and hence isexcellent in durability. Thus, a specific configuration example of alithium ion secondary battery using the electrode 10 in accordance withthe present embodiment will be described hereinafter with reference tothe accompanying drawings.

A lithium ion secondary battery 100 shown in FIG. 2 is a sealed typebattery constructed by accommodating a flat-shaped wound electrode body20 and a non-aqueous electrolyte (not shown) in a flat square batterycase (i.e., an exterior container) 30. The battery case 30 is providedwith a positive electrode terminal 42 and a negative electrode terminal44 for external connection, and a thin-walled safety valve 36 set so asto relieve the internal pressure when the internal pressure of thebattery case 30 increases to a prescribed level or higher. The positiveand negative electrode terminals 42 and 44 are electrically connectedwith positive and negative electrode collector plates 42 a and 44 a,respectively. As the material for the battery case 30, for example, ametal material which is lightweight, and has a good thermal conductivitysuch as aluminum is used.

For the wound electrode body 20, as shown in FIGS. 2 and 3, a positiveelectrode sheet 50 includes a positive electrode active material layer54 formed along the longitudinal direction on one surface or bothsurfaces (both surfaces in this case) of a long-length positiveelectrode collector 52; and a negative electrode sheet 60 includes anegative electrode active material layer 64 formed along thelongitudinal direction on one surface or both surfaces (both surface inthis case) of a long-length negative electrode collector 62. Thepositive electrode sheet 50 and the negative electrode sheet 60 arestacked and wound in the longitudinal direction with two long-lengthseparator sheets 70 interposed therebetween. It should be noted that apositive electrode collector plate 42 a and a negative electrodecollector plate 44 a are bonded to a positive electrode active materiallayer non-formation part 52 a (i.e., the portion of the positiveelectrode collector 52 exposed without formation of the positiveelectrode active material layer 54 thereon), and a negative electrodeactive material layer non-formation part 62 a (i.e., the portion of thenegative electrode collector 62 exposed without formation of thenegative electrode active material layer 64 thereon) that are formed soas to extend off outwardly from both ends in the winding axial direction(denoting the sheet width direction orthogonal to the longitudinaldirection) of the wound electrode body 20, respectively.

For each of the positive electrode sheet 50 and the negative electrodesheet 60, the electrode 10 in accordance with the present embodiment isused.

In the present example, for the positive electrode sheet 50, aluminumfoil is used as the positive electrode collector 52, and the positiveelectrode active material layer 54 includes a positive electrode activematerial, a carbon nanotube, and a binder. The carbon nanotube has thecoating mentioned above.

In the present example, for the negative electrode sheet 60, copper foilis used as the negative electrode collector 62, and the negativeelectrode active material layer 64 includes graphite of a negativeelectrode active material, a carbon nanotube, a binder, and a thickener.The carbon nanotube has the coating mentioned above.

It should be noted that, in the present example, for both the positiveelectrode sheet 50 and the negative electrode sheet 60, the electrodes10 in accordance with the present embodiment are used. As for thelithium ion secondary battery 100, the electrode 10 in accordance withthe present embodiment may be used for only one of the positiveelectrode sheet 50 and the negative electrode sheet 60. However, use ofthe electrodes 10 in accordance with the present embodiment for both thepositive electrode sheet 50 and the negative electrode sheet 60 canprovide a larger resistance characteristics improving effect.

As the separators 70, the same various microporous sheets as thoseconventionally used for a lithium ion secondary battery can be used, andexamples thereof may include a microporous resin sheet formed of a resinsuch as polyethylene (PE) or polypropylene (PP). Such a microporousresin sheet may be of a monolayer structure, or may be a multilayeredstructure of double layer or more (e.g., a three-layered structure inwhich PP layers are stacked on both surfaces of a PE layer). Theseparator 70 may include a heat resistant layer (HRL).

As the non-aqueous electrolytic solution, the same ones as those for aconventional lithium ion secondary battery are usable. Typically, theone obtained by allowing an organic solvent (non-aqueous solvent) toinclude a support salt is usable. As the non-aqueous solvents, variousorganic solvents such as carbonates, ethers, esters, nitriles, sulfones,and lactones for use in the electrolytic solution of a general lithiumion secondary battery can be used without particular restriction.Specific examples thereof may include ethylene carbonate (EC), propylenecarbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (MFEC),difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethylcarbonate (F-DMC), and trifluoro dimethyl carbonate (TFDMC). Suchnon-aqueous solvents may be used alone, or in appropriate combination oftwo or more thereof. As the support salt, for example, lithium salt suchas LiPF₆, LiBF₄, or LiClO₄ (desirably LiPF₆) can be desirably used. Theconcentration of the support salt is desirably 0.7 mol/L or more and 1.3mol/L or lower.

It should be noted that the non-aqueous electrolytic solution mayinclude, for example, various additives of gas generator such asbiphenyl (BP) or cyclohexyl benzene (CHB); film forming agent such asoxalato complex compound including a boron atom and/or phosphorus atomor vinylene carbonate (VC); dispersant; thickener; and other additivesso long as the effects of the present teaching is not remarkablyimpaired.

The lithium ion secondary battery 100 is usable for various uses. Thedesirable uses may include a driving power source to be mounted in avehicle such as a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV),or an electric vehicle (EV). The lithium ion secondary batteries 100 canbe used in a form of a battery back in which a plurality thereof areelectrically connected with one another.

It should be noted that, as one example, the rectangular lithium ionsecondary battery 100 including the flat-shaped wound electrode body 20was described. However, a lithium ion secondary battery can also beconfigured as a lithium ion secondary battery including a stacked typeelectrode body. Alternatively, a lithium ion secondary battery can alsobe configured as a cylindrical lithium ion secondary battery, a laminatetype lithium ion battery, or the like.

Alternatively, according to a known method, using the electrode 10 inaccordance with the present embodiment, other secondary batteries(particularly, a non-aqueous electrolytic solution secondary battery)other than a lithium ion secondary battery can also be configured.

Examples regarding the present teaching will be described in detailshereinafter. However, it is not intended that the present teaching islimited to such examples.

Manufacturing of Coated Carbon Nanotube

A dispersion of an oxide of the element shown in Table 1 (e.g., titaniumoxide in the case of Ti) was prepared. To the dispersion, a small amountof acrylic resin was dissolved as a binder. Into the dispersion, acarbon nanotube having the average length shown in Table 1, andmanufactured by a chemical vapor deposition method was immersed.Subsequently, evaporation to dryness was performed, resulting in acarbon nanotube coated with an oxide of the element shown in Table 1. Ithas been indicated by cross-section EDS mapping that the element shownin Table 1 coats the carbon nanotube. Further, the coverage rate wascalculated from (total of deposition distances of element(s))/(entirecircumferential length of carbon nanotube)×100. The results are shown inTable 1.

Study of Positive Electrode: Examples 1 to 16 and Comparative Example 1Manufacturing of Positive Electrode

LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ (LNCM) as a positive electrode activematerial powder, acetylene black (AB) as a conductive material, themanufactured coated carbon nanotube (CN), and polyvinylidene fluoride(PVDF) as a binder were mixed with N-methyl pyrrolidone (NMP) at a massratio of LNCM:AB:CN:PVDF=94:1.5:1.5:3, thereby preparing a positiveelectrode active material layer forming paste. The paste was coated in aband shape on both surfaces of aluminum foil with a thickness of 15 μm,and was dried and subjected to a press treatment, thereby manufacturinga positive electrode.

Manufacturing of Battery for Evaluation

Spheroidized graphite (C) as a negative electrode active material, CMCas a thickener, and SBR as a binder were mixed with ion exchanged waterat a mass ratio of C:CMC:SBR=98:1:1, thereby preparing a negativeelectrode active material layer forming paste. The paste was coated in aband shape on both surfaces of copper foil with a thickness of 10 μm,and was dried and subjected to a press treatment, thereby manufacturinga negative electrode.

Further, two separators (porous polyolefine sheets of a three-layeredstructure of PP/PE/PP with a thickness of 20 μm) were prepared.

The manufactured positive electrode and negative electrode, andseparators were laminated so that the separators were interposed betweenthe positive and negative electrodes, and the resultant laminate waswound. Thus, a wound electrode body was obtained.

The manufactured wound electrode body was accommodated in a batterycase.

Subsequently, a non-aqueous electrolytic solution was injected into thebattery case, thereby manufacturing a rectangular lithium ion secondarybattery with a capacity of 5 Ah. It should be noted that, for thenon-aqueous electrolytic solution, there was used the one obtained bydissolving LiPF₆ as a support salt at a concentration of 1.0 mol/L in amixed solvent including ethylene carbonate (EC), dimethyl carbonate(DMC), and ethyl methyl carbonate (EMC) at a volume ratio ofEC:DMC:EMC=3:4:3.

In the manner as described above, lithium ion secondary batteries forevaluation using electrodes of Examples 1 to 16 were obtained.

Meanwhile, a lithium ion secondary battery for evaluation using theelectrode of Comparative Example 1 was obtained in the same manner asdescribed above, except for using a carbon nanotube as it was (i.e.,using an uncoated carbon nanotube).

Study of Negative Electrode: Examples 17 to 19 and Comparative Examples2 to 4 Manufacturing of Negative Electrode

The negative electrode active material (NA) shown in Table 1, themanufactured coated carbon nanotube (CN), CMC as a thickener, and SBR asa binder were mixed with ion exchanged water at a mass ratio ofNA:CN:CMC:SBR=96.5:1.5:1:1, thereby preparing a negative electrodeactive material layer forming paste. The paste was coated in a bandshape on both surfaces of copper foil with a thickness of 10 μm, and wasdried and subjected to a press treatment, thereby manufacturing anegative electrode.

Manufacturing of Battery for Evaluation

LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ (LNCM) as a positive electrode activematerial powder, acetylene black (AB) as a conductive material, andpolyvinylidene fluoride (PVDF) as a binder were mixed with N-methylpyrrolidone (NMP) at a mass ratio of LNCM:AB:PVDF=94:3:3, therebypreparing a positive electrode active material layer forming paste. Thepaste was coated in a band shape on both surfaces of aluminum foil witha thickness of 15 μm, and was dried and subjected to a press treatment,thereby manufacturing a positive electrode.

Further, two separators (porous polyolefine sheets of a three-layeredstructure of PP/PE/PP with a thickness of 20 μm) were prepared.

The manufactured positive electrode and negative electrode, andseparators were laminated so that the separators were interposed betweenthe positive and negative electrodes, and the resultant laminate waswound. Thus, a wound electrode body was obtained.

The manufactured wound electrode body was accommodated in a batterycase. Subsequently, a non-aqueous electrolytic solution was injectedinto the battery case, thereby manufacturing a rectangular lithium ionsecondary battery with a capacity of 5 Ah. It should be noted that, forthe non-aqueous electrolytic solution, there was used the one obtainedby dissolving LiPF₆ as a support salt at a concentration of 1.0 mol/L ina mixed solvent including ethylene carbonate (EC), dimethyl carbonate(DMC), and ethyl methyl carbonate (EMC) at a volume ratio ofEC:DMC:EMC=3:4:3.

In the manner as described above, lithium ion secondary batteries forevaluation using electrodes of Examples 17 to 19 were obtained.

Meanwhile, lithium ion secondary batteries for evaluation using theelectrodes of Comparative Examples 2 to 4 were obtained in the samemanner as described above, except for using a carbon nanotube as it was(i.e., using an uncoated carbon nanotube).

Initial Resistance Evaluation

Each lithium ion secondary battery for evaluation was adjusted to astate of charge (SOC) of 50%, and then, was allowed to stand under 25°C. environment. At a current value of 100 Ah, 10-second discharging wasperformed. The voltage value after 10 seconds from start of dischargingwas measured, and the initial battery resistance was calculated. Theratio of each resistance of other lithium ion secondary batteries forevaluation when the resistance of the lithium ion secondary battery forevaluation of Comparative Example 1 was taken as 100 was determined(i.e., the ratio was determined in terms of percentage). The results areshown in Table 1.

Evaluation of Resistance after High-temperature Storage

Each lithium ion secondary battery for evaluation was adjusted to a SOCof 100%, and was stored under 60° C. environment for 30 days. Then, thebattery resistance was measured in the same manner as described above.The ratio of each resistance of other lithium ion secondary batteriesfor evaluation when the resistance of the lithium ion secondary batteryfor evaluation of Comparative Example 1 was taken as 100 was determined(i.e., the ratio was determined in terms of percentage). The results areshown in Table 1.

TABLE 1 CN Resistance containing CN-coated CN ratio after electrodeelement average Initial high- (active (electro- length Coverageresistance temperature material) negativity) (μm) rate (%) ratio (%)storage (%) Ex. 1 Positive Ti (1.54) 10 50 70 75 Ex. 2 electrode P(2.19) 77 80 Ex. 3 (LNCM) B (2.04) 75 75 Ex. 4 Si (1.90) 75 85 Ex. 5 Al(1.61) 73 85 Ex. 6 Zn (1.65) 73 80 Ex. 7 W (2.36) 80 85 Ex. 8 Ti (1.54)1 85 90 Ex. 9 3 75 87 Ex. 10 50 72 74 Ex. 11 100 85 90 Ex. 12 10 30 8580 Ex. 13 40 75 77 Ex. 14 70 75 74 Ex. 15 80 85 72 Ex. 16 Ti (1.54), P(2.19) 50 73 77 Comp. Without coating 100 100 Ex. 1 Ex. 17 Negative Ti(1.54) 10 50 108 75 Comp. electrode Without coating 110 100 Ex. 2(graphite) Ex. 18 Negative Ti (1.54) 10 50 75 70 Comp. electrode Withoutcoating 115 95 Ex. 3 (LTO) Ex. 19 Negative Ti (1.54) 10 50 80 80 Comp.electrode Without coating 120 105 Ex. 4 (Si) Ex.: Example Comp. Ex.:Comparative example

The results of Examples 1 to 7 and Comparative Example 1 indicate thatuse of a carbon nanotube coated with a material including an elementwith a lower electronegativity than that of carbon reduces the initialresistance, and also suppresses the increase in resistance afterhigh-temperature storage.

Results of Example 1 and Examples 8 to 11 indicate that when the averagelength of carbon nanotubes is 3 μm or more and 50 μm or less, theresistance characteristics improving effect is particularly high. Thisis considered due to the following: when the length of the carbonnanotube is reduced, the conductive path tends to become less likely tobe formed; and an increase in length of the carbon nanotube causesaggregation of the carbon nanotubes, so that the resistance reducingeffect tends to be reduced.

The results of Example 1 and Examples 12 to 15 indicate that when thecoverage rate is 40% or more and 70% or lower, the resistancecharacteristics improving effect is particularly high. This isconsidered due to the following: a decrease in coverage rate tends toreduce the resistance characteristics improving effect due to coating;and an increase in coverage rate increases the influence of insulatingthe point of contact between the active material and the carbonnanotube.

The results of Example 16 indicate that even when the material forcoating the carbon nanotube included a plurality of elements with alower electronegativity than that of carbon, the resistancecharacteristics improving effect was observed.

The results of Examples 17 to 19 and Comparative Examples 2 to 4indicates that even when a carbon nanotube coated with a materialincluding an element with a lower electronegativity than that of carbonwas used for the negative electrode, the initial resistance was reduced,and the increase in resistance after high-temperature storage wassuppressed. When the negative electrode active materials were LTO andSi, the initial resistance reducing effect was large. In contrast, whengraphite was used for the negative electrode active material, the degreeof reduction of the initial resistance was small. This is considered dueto the fact that the electron conductivity of graphite was sufficientlyhigh, and the room for improvement of the electron conductivity wassmall.

Up to this point, specific examples of the present teaching weredescribed in details. However, these are merely examples, and do notlimit the scope of the appended claims. The technology described in theappended claims includes various modifications and changes of thespecific examples exemplified up to this point.

What is claimed is:
 1. An electrode for a secondary battery, comprising:a collector; and an active material layer formed on the collector,wherein the active material layer includes an active material and acarbon nanotube, and at least a part of a surface of the carbon nanotubeis coated with a material including an element with a lowerelectronegativity than that of carbon, and wherein the electrode for asecondary battery is a negative electrode, and the active material isLi₄Ti₅O₁₂ or Si.
 2. The electrode for a secondary battery according toclaim 1, wherein the element with a lower electronegativity than that ofcarbon is at least one selected from the group consisting of Ti, P, B,Si, Al, Zn, and W.
 3. The electrode for a secondary battery according toclaim 1, wherein the average length of the carbon nanotubes is 3 μm ormore and 50 μm or less.
 4. The electrode for a secondary batteryaccording to claim 1, wherein the coating ratio of the surface of thecarbon nanotube with the material including an element with a lowerelectronegativity than that of carbon is 40% or more and 70% or lower.5. The electrode for a secondary battery according to claim 1, whereinthe material including an element with a lower electronegativity thanthat of carbon is an oxide of the element with a lower electronegativitythan that of carbon.
 6. A secondary battery comprising the electrode fora secondary battery according to claim 1.